2.5 Troubleshoot Network Media

Short

An electrical short occurs when electrical signals take a path other than the intended path. In the case of twisted pair wiring, a short means that a signal sent on one wire arrives on a different wire.

Short

An electrical short occurs when electrical signals take a path other than the intended path. In the case of twisted pair wiring, a short means that a signal sent on one wire arrives on a different wire. Shorts occur when two wires touch; this can be caused by worn wire jackets, crushed wires, or a metal object piercing two or more wires.

Incorrect Termination

An incorrect termination occurs when an incompatible or incorrect connector is used. This can result in reduced performance or complete connection loss.

Open Circuit

An open circuit is when a cut in the wire prevents the original signal from reaching the end of the wire. An open circuit is different from a short in that the signal stops (electricity cannot flow because the path is disconnected).

Attenuation

Attenuation is the loss of signal strength from one end of a cable to the other. This is also known as dB loss.

Attenuation

Attenuation is the loss of signal strength from one end of a cable to the other. This is also known as dB loss. > The longer the cable, the more attenuation. For this reason, it is important never to exceed the maximum cable length defined by the networking architecture. > Cables at a higher temperature experience more attenuation than cables at a lower temperature. > A repeater regenerates the signal and removes the effects of attenuation.

Cable Certifier

> A cable certifier is a multi-function tool that verifies that a cable or an installation meets the requirements for a specific architecture implementation. For example, you would use a certifier to verify that a specific drop cable meets the specifications for 1000BaseT networking. > A certifier is very important for Cat 6 cable used with bandwidths at or above 1000 Mbps. Slight errors in connectors or wires can cause the network to function at 100 Mbps instead of the desired 1000 Mbps or 10 Gbps. > Certifiers can also validate the bandwidth capabilities of network interface cards and switches. Many can detect the duplex settings of network devices. > Most certifiers include features of a toner probe, TDR, and cable tester. > Certifiers are very expensive and are typically used by organizations that specialize in wiring installations. The following image shows some outputs you might see when using a cable certifier:

Cable Tester

> A cable tester (or line tester) verifies that the cable can carry a signal from one end to the other and that all wires are in the correct positions. - High-end cable testers can check for various miswire conditions (wire mapping, reversals, split pairs, shorts, or open circuits). - You can use a cable tester to quickly tell the difference between a crossover and a straight-through cable.. - Most testers have a single unit that tests both ends of the cable at once. Many testers come with a second unit that can be plugged into one end of a long cable run to test the entire cable.

Loopback Plug

> A loopback plug, or loopback adapter, reflects a signal from the transmit port on a device to the receive port on the same device. Use the loopback plug to verify that a device can both send and receive signals. > There are loopback plugs for both copper and fiber connections. > A failure in the loopback test indicates a faulty network card. > A successful loopback test means the problem is in the network cabling or another connectivity device. > You can purchase pre-made loopback plugs, or you can make an inexpensive one by cutting the end off a cable and manually connecting the transmit wires to the receive wires (connect the wire from pin 1 to the wire at pin 3, and the wire at pin 2 to the wire at pin 6).

Multimeter

> A multimeter is a device used to test various electrical properties. A multimeter can measure several parameters: - AC and DC voltage - Current (amps) - Resistance (ohms) - Capacitance - Frequency

Time-Domain Reflectometer (TDR)

> A time-domain reflectometer is a special device that sends electrical pulses on a wire in order to discover information about the cable. The TDR measures impedance discontinuities (the echo received on the same wire in response to a signal on the wire). The results of this test can be used to identify several variables: > Estimated wire length > Cable impedance > The location of splices and connectors on the wire > The location of shorts and open circuits

Toner Probe

> A toner probe is composed of two devices that are used together to trace the end of a wire from a known endpoint to the termination point in the wiring closet. To use a toner probe: - Connect the tone generator to one end of the wire. It will send a signal on the wire. - In the wiring closet, touch the probe to wires or place the probe close to wires. A sound at the probe indicates that the generated tone has been detected and the wire that you are touching is the termination point for the wire you are tracing.

Voltage Event Recorder

> A voltage event recorder keeps track of voltage conditions on a power line. Basic recorders simply keep track of the occurrence of undervoltage or overvoltage conditions, while more advanced devices track conditions over time and create a graph, saving data from a program running on a computer. Some UPS systems include a simple voltage event recorder. Use a voltage event recorder to identify periods of low or high voltage that can adversely affect network components.

Environmental Monitor

> An environmental monitor does what its name implies—it monitors the environmental conditions of a specific area or device. - Monitors are often used to track the conditions within server rooms (such as temperature, humidity, water, smoke, motion, and air flow). - Typically computers, especially servers, have an internal monitor that measures fan speed and CPU temperature. - Many monitors will sound an alarm if a specified temperature or other environmental condition is reached.

Optical Time-Domain Reflectometer (OTDR)

> An optical time-domain reflector performs the same function as a TDR, but is used for fiber optic cables. An OTDR sends light pulses into the fiber cable and measures the light that is scattered or reflected back to the device. The information is then used to identify specifics about the cable: > The location of a break > Estimated cable length > Signal attenuation (loss) over the length of the cable

As you study this section, answer the following questions:

> How do you prevent back reflection and optical return loss? > What is the difference between a short circuit and an open circuit? > What happens when you connect a single mode fiber to multimode fiber? > What is the difference between a time-domain reflectometer and an optical time-domain reflectometer? > Which tool would you use to test the bandwidth of your internet connection? > Which cable types are immune to the effects of EMI? > How does distance affect attenuation? How does distance affect impedance? > What is the single best method to reduce the effects of an impedance mismatch? > What is the difference between a regular cable tester and a cable certifier? > Which tool would you use to find the end of a specific cable within a wiring closet?

Known Good Spares

> One valuable troubleshooting method is to keep a set of components that you know are in proper functioning order. If you suspect a problem in a component, swap it with the known good component. If the problem is not resolved, troubleshoot other components. Examples of using this strategy include the following: - Changing the drop cable that connects a computer to the network - Replacing a NIC with a verified working NIC - Moving a device from one switch port to another

Fiber Optic Connectors

> Several issues can occur when you are working with fiber optic connectors. > For light to pass through a fiber optic connector, the fiber within the jack must line up perfectly with the fiber in the connector. Using the wrong connector will result in misaligned fibers, disrupting the light signal even if the connector is successfully locked into the jack. > Dirty connectors can also impede or disrupt the light signal, so it's important that they are kept clean. Several cleaning methods can be used with fiber optic connectors: - For connectors where the ferrule protrudes out of the connector, such as the FC connector, you can wipe the end of the ferrule with a lint-free cloth that has a small amount of denatured alcohol applied. Immediately wipe the ferrule dry with a dry lint-free cloth. - For fiber optic connectors where the end of the ferrule is less accessible, you must use a specialized cleaning tool. Some cleaning tools allow you to plug in the fiber optic cable and then clean it by pumping the tool's handle. - To clean the jacks on fiber-optic network interfaces, you can purchase a specialized fiber optic cleaning stick to remove foreign material.

Fiber Optic Polishing

> Whenever a connector is installed on the end of fiber optic cable, a degree of signal loss occurs. This is called insertion loss. Additionally, some of the light that is lost is reflected directly back down the cable, toward the source. This is called back-reflection, or optical return loss (ORL). It can corrupt the data being transmitted and even damage the transmitter. > For a connector to work properly, there must be as little insertion loss and ORL as possible. The better the polish on the connector, the better the light will pas through without reflection. Fiber optic equipment manufacturers rate their connectors using the following polish grading designations: > Physical contact (PC) polishing is usually used with single-mode fiber. The ends of the fiber are polished with a slight curvature so that when the cable end is inserted into the connector, only the cores of the fiber actually touch each other. > Super physical contact (SPC) and ultra physical contact (UPC) polishing use a higher grade of polish and have more of a curvature than PC polishing, further reducing ORL reflections. > Angled physical contact (APC) polishing is used to reduce back reflection as much as possible. An APC connector has an eight-degree angle cut into the ferrule, which prevents reflected light from traveling back down the fiber. Any reflected light is bounced out into the cable cladding instead. You can only use angle-polished connectors with other angle polished connectors. Using an angle-polished connector with a non-angle-polished connector causes excessive insertion loss. * APC connectors are colored green to prevent them from being mixed with non-APC connectors.

Wire Stripper, Snips, and Crimpers

> Wire strippers remove the protective sheath of a cable in order to expose the conductive wire. - Wire strippers are rated to specific gauge (cable width) ranges. - Most wire strippers are combination tools (they can strip, cut, and crimp cables). - Almost all wire strippers have multiple holes or can be adjusted for specific cable sizes. > A crimping tool is used to attach connectors to wires. Some crimpers are designed for power connections. A special crimper is used to attach RJ45 connectors to twisted pair cables. > Snips are cutting tools used to cut cables or wires to a specific length or to remove damaged sections. A diagonal cutter is an example of a snip tool. *Whenever possible, use a wire stripper instead of snips to strip a cable. Wire strippers are specifically designed to cut only the protective sheath without cutting the internal wire.

Bad Connector

A bad connector is a damaged connector that is causing connectivity issues. For example, a broken locking tab on an RJ45 connector can cause intermittent connection problems. Another common connector is when there is a bent or damaged pin, especially on female RJ4 connections and the center wire of a coaxial cable.

Mis-wired Cable

A miswired cable is caused by incorrect wire positions on both connectors. Several wiring problems might exist: > A reverse connection is when a cable is wired using one standard on one end and another standard on the other end, creating a crossover cable. While this condition might be intentional, it can cause problems when a crossover cable is used instead of a straight-through cable. This will reverse the transmit and receive match up; the transmit pins on one end maps to the transmit pins on the other end when they should map to the receive pins on the other end. > Wiremapping is matching a wire with a pin on one end with the same pin on the other end. For example, an error in the wiremapping results when the wire at pin 1 connects to pin 4. A split pair condition is when a single wire in two different pairs is reversed at both ends. For example, if instead of the solid green wire, the solid brown wire is matched with the green/white wire in pins 1 and 2. With a split pair configuration, the cable might still work (especially if it is short), but it could introduce crosstalk. - When the T568A/B standards for making drop cables are followed, one pair is split to meet the standards. In this case, a common split pair error is simply placing all wire pairs in order in the connector instead of splitting the pair according to the standard. - When cables are connected using a punch down block, pairs are not split.

Smartjack

A smartjack is an intelligent loopback device installed at the demarcation point for a WAN service. Technicians at the central office can send diagnostic commands to the smartjack to test connectivity and performance between the central office and the demarc. When you contact a WAN service provider for assistance, they might execute a test using the smartjack. A successful test indicates that the problem is within your customer premises equipment (CPE).

Speed Test Website

A speed test website is an online tool that is used to test the bandwidth of your internet connection. There are countless speed test websites available, all of which provide essentially the same information: - Connection latency (ping) - Download speed - Upload speed

Crosstalk

Crosstalk is interference that is caused by signals within the twisted pairs of wires (for example, current flow on one twisted pair causing a current flow on an adjacent pair). > The twisting of wires into pairs helps reduce crosstalk between pairs. > Each pair of wires is twisted at a different rate to reduce crosstalk between pairs. > Crosstalk is often introduced within connectors, where the twists are removed to add the connector. Crosstalk can also occur where wires are crushed or where the plastic coating is worn. There are several forms of crosstalk: > Near-end crosstalk (NEXT) is measured on the same end as the transmitter. For example, when a signal is sent on one wire pair, near-end crosstalk measures the interference on an adjacent wire pair at the same connector end. > Far-end crosstalk (FEXT) is measured on the end without the transmitter. For example, when a signal is sent on one wire pair, far-end crosstalk measures the interference on an adjacent wire pair at the opposite connector end. > Alien crosstalk is introduced from adjacent, parallel cables. For example, a signal sent on one wire pair causes interference on a wire pair that is within a separate twisted pair cable bundle.

Crosstalk

Crosstalk is interference that is caused by signals within twisted pairs of wires (for example, current flow on one twisted pair causing a current flow on an adjacent pair).

Electromagnetic Interference (EMI)andRadio Frequency Interference (RFI)

Electromagnetic interference and radio frequency interference are external signals that interfere with normal network communications. Common sources of EMI/RFI include nearby generators, motors (such as elevator motors), radio transmitters, welders, transformers, and fluorescent lighting.

Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI)

Electromagnetic interference and radio frequency interference are external signals that interfere with normal network communications. Common sources of EMI/RFI include nearby generators, motors (such as elevator motors), radio transmitters, welders, transformers, and fluorescent lighting. To protect against EMI/RFI: > Use fiber optic instead of copper cables. Fiber optic cables are immune to EMI/RFI. > Use shielded twisted pair cables. Shielded cables have a metal foil that encloses all of the wires. Some cables might also include a drain wire that is a bare wire outside of the foil, but within the cable jacket. The drain wire can be grounded to help absorb EMI/RFI. > Avoid installing cables near EMI/RFI sources.

Open Impedance Mismatch (Echo)

Impedance is the measure of resistance within the transmission medium. > Impedance is measured in ohms (Ω). > All cables must have the same impedance rating. The impedance rating for the cable must match the impedance of the transmitting device. > Impedance is mostly a factor in coaxial cables used for networking. Be sure to choose a cable with the correct rating (50 or 75 ohm) based on the network type. Never mix cables with different ratings. > When signals move from a cable with one impedance rating to a cable with another rating, some of the signals is reflected back to the transmitter, distorting the signal. With video (cable TV), impedance mismatch is manifested as ghosting of the image. > Cable distance does not affect the impedance of the cable.

Signal Loss

Light signals being transmitted through a fiber optic cable experience attenuation, or signal loss, as they pass through the cable due to: > Reflection: A measurable amount of light is reflected when it hits the ends of the cable. Much of a cable's reflection loss occurs at each cable connection. When the light hits the boundary between the core and the cladding, it is reflected back into the core. There is minor loss to the signal when this occurs, but it contributes to overall signal loss. > Refraction: If the light hits the boundary between the core and the cladding at too steep of an angle, the light is refracted into the cladding instead of reflected back into the core, causing signal loss. Some fiber optic cables are doped with impurities near the edge of the fiber so that the signals are bent instead of reflected back to the center of the core. The loss due to this refraction is minor when compared with the benefits of confining the light to the center of the core. > Scattering: Impurities in the fiber core can cause light to scatter. Some of the light continues down the fiber. The light that is scattered backwards contributes to the signal loss. > Absorption: Impurities in the fiber can also absorb the light, converting it to another form of energy, like heat. This is a major cause of signal loss. Several physical cable attributes can contribute to signal loss: > Cable length: while higher quality cables will carry light signals further, the longer the cable, the more signal absorption and the greater the signal loss. > Connectors: every connector will cause some level of signal loss, mostly due to reflection. While patch cables at each end of a run are to be expected, you should minimize any other connections. > Splices: there are tools that you can use to splice a cut fiber optic cable. However, the signal loss from a splice is comparable to the signal loss from a connector. > Bends: micro bends in the cable due to things such as temperature change or manufacturing anomalies can cause signal loss. While you have little control over micro bends, even macro bends, those detected by the human eye, can contribute to signal loss. The straighter the fiber optic cable, the less the signal loss. > You can estimate how much signal loss (measured in dB) you should reasonably expect in a given run of fiber optic cabling. Signal loss is calculated by summing the average loss of all the components used in the cable run to generate an estimate of the total attenuation that will be experienced end-to-end. This estimate is called a loss budget. > When calculating a loss budget for a segment of fiber optic cable, use the following guidelines: - Connectors: 0.3 dB loss each - Splices: 0.3 dB loss each > Multi-mode cabling: > 3 dB loss per 1000 meters when using an 850 nm light source > 1 dB loss per 1000 meters when using a 1300 nm light source > Single-mode cabling: - 0.5 dB loss per 1000 meters when using a 1310 nm light source - 0.4 dB loss per 1000 meters when using a 1550 nm light source > The total attenuation should be no more than 3 dB less than the total power at the transmission source. This is called the link-loss margin. For example, if the total power output at the transmission source of a cable run is 15 dB, then the total attenuation over the cable run should not exceed 12 dB. This ensures that the cable will continue to function as its components (such as the LED light transmitters and connectors) degrade with age and use.

Media Adapters and Transceivers

Many network switches and routers allow you to insert a transceiver such as a gigabit interface converter (GBIC) in an empty slot to convert the interface from copper wiring to fiber optic. Other devices use a small form-factor pluggable (SFP) transceiver to accomplish the same goal. Several issues can occur when using these and other fiber optic media adapters: > Some GBIC/SFP modules use multimode fiber, while others use single-mode. Make sure that you use the correct type of fiber optic cable and connector required by the specific adapter. > Media adapter modules malfunction on occasion. If you have lost connectivity on one of these links, ensure that the adapter module is working correctly.

Damaged and Mismatched Cables

Several issues can occur when you are working with fiber optic cabling. > Fiber optic cabling is much less forgiving of physical abuse than copper wiring. The fiber core is fragile and can be easily damaged by rough handling. For example, bending a fiber cable at too tight of a radius will break the core. > Wavelength mismatch will cause serious issues with fiber optic cables. You cannot mix and match different types of cable. For example, if you connect single-mode fiber to multi-mode fiber, you will introduce a catastrophic signal loss of up to 99%. Even connecting cables of the same type that have different core diameters can cause a loss of up to 50% of the signal strength.

2.5.2 Copper Wiring Troubleshooting Facts

The following table describes several conditions that can affect the performance and functions of copper wiring.

2.5.4 Fiber Optic Wiring Troubleshooting Facts

Troubleshooting fiber optic wiring is more complex than troubleshooting copper network wiring. To function properly, fiber optic cabling must be created, installed, and maintained very carefully. Several important factors that can affect fiber optic performance are listed in the following table.

2.5.6 Troubleshooting Tools Facts

Troubleshooting physical connectivity problems is much easier if you use the appropriate troubleshooting tool.

Transceiver Mismatch

Well-manufactured network devices have interfaces that can be tailored to different cable types, protocols, and speeds. This is done by connecting a hot swappable transceiver to the interface. When connecting one network device to another, matching transceivers must be used. For twisted pair cabling, a mismatch in speed is a common issue.